The Plant-Ant Connection

Author: Wendy Benfer, Kingaroy Branch

Published: Journal March 2022 

This is a story of a relationship between two shrubs and some ants, and their dependence on each other for healthy survival.

The two shrubs are the Canthiums, Psydrax odorata forma australiana and Psydrax odorata forma buxifolia. It has been noted that these are rarely found without ant colonies. When the shrub is about 45cm high, the queen of a Podomyrma species of ant, from a neighbouring Canthium bush, begins to set up a colony in the young shrub. It does this by boring into certain twigs, on the end of the newforming branches, which have soft centre segments. The winged queen cuts out circular entrance holes on these twigs and hollows out the pithy centre. A colony is then established inside the hollowed out twig. This colony persists for the life of the twig.

The ants keep two types of insects inside their chambers, namely mealy bugs and scale insects. The mealy bugs are mobile and fluffy, moving around inside the chamber while the scale insects are smooth and fixed in position in pits which the ants gnaw into the inner walls of their chambers. These insects, protected from predators by the ants, produce copious amounts of honey-dew, a food item prized by the ants. The plant benefits because the ants help to keep destructive, plant-eating insects away.

Psydrax plants belong to the family, Rubiaceae, the same plant family which contains ant plants and trees in Africa and South America where the obligatory relationship between the trees and certain ants that nest in their cavities is well known. The ants associated with our Canthium bushes are found on these plants exclusively.

Reprinted with permission from the Kingaroy Branch Newsletter, October 2021 and originally published in ‘Meet the Locals’, The Country Focus, April 2013